1. Field of the Invention
This invention concerns a process for the controlled burning of a stack of solid fuel, particularly wood, which is located in a vertical furnace shaft, wherein the solid fuel is ignited at its top and allowed to burn downwardly to its bottom.
2. The Prior Art
In furnaces used for burning solid fuels, especially wood, it is very difficult to attain an even and hygienic combustion of a single charge of the solid fuel over a time period of several hours. In the known furnaces for burning solid fuels which have a vertical furnace shaft, the combustion is usually commenced at the bottom of the vertical stack of solid fuel. This means that the solid fuel (wood) is first stacked in the furnace shaft, the floor of which is constructed as grate, and the wood stack is then lit at its bottom. The combustion air needed for the combustion is led in from below through the grate and the combustion is led away at its side. Theoretically, the wood stack should burn evenly from the bottom and the fuel should slip down evenly from the top. However, in reality a burn-out-like condition occurs after a relatively short time, i.e., the total amount of wood in the furnace shaft becomes thermally decomposed, with the result being very high, but timewise limited, output peak, or else an intensive smoke development in the case where the waste gases cannot escape from the top. The reason for this is that a convective heat exchange occurs with the fuel located above the embers or that the hot waste gases resulting from the combustion heat up the wood above the bed of embers on their way up, i.e., in the event that exhaust of the combustion gases above the fuel is possible. However, according to the known state of the art, wood is already thermally decomposed at temperatures between 100.degree. and 150.degree. C., whereby combustible and easily flammable gases are formed which then lead in a short time to the noted reaction by the whole wood stack. Thus, a controlled, continuous and hygienic combustion is not attained.
For the foregoing reason, attempts have been made to work with a so-called combustion from the top, in which the wood stack is lit at its top surface so as to burn down from its top. However, in reality a burn-out of the whole wood stack occurs in a comparably short time. In this regard, the speed of flow of the combustion gases, fed in from the bottom, i.e., through the grate, through the wood stack to the top to the bed of embers is very small, so that the combustion air is heated far below the bed of embers because of the comparatively rapid progress of the heat transfer, with the consequence that the decomposition processes of the wood, as described above in connection with combustion from the bottom, occur here too, in this case progressing from top to bottom. Thus, the wood stack ignites from top to bottom and there is a burn-out as in the case of combustion from the bottom. Compared to the operation with combustion from the bottom, there is thus only the difference that the undesirable high output peak (burn-out) takes place with a certain time delay.
Though it is known from German Pat. No. 33 292 and German Patent Publication No. 31 42 394 to feed in combustion air from the side and to have it follow the embers, the combustion process takes place, however, from the end of the stack removed from the waste gas discharge, with the consequence that the hot waste gases have to flow through the unignited areas of the stack. Finally, it is known from German Patent Publication No. 28 04 968 for a boiler with top waste gas release and combustion from top to bottom, to lead combustion air from the top to the stack through a shiftable pipe and to have the pipe follow the continuous combustion of the stack towards the bottom. However, this blowing on the embers from the top requires the combustion air to be blown in under a comparatively high pressure since it must flow counter to the direction of flow of the escaping waste gases. In this manner, however, hot carbonization gases in the stack are pressed towards the bottom, which not only promotes a tarring of the lower parts of the combustion chamber but overall leads to an uncontrolled combustion because a lighting of the fuel by the noted carbonization gases. In addition, this known process is not suitable for individual furnaces because of the mechanical devices needed to control the lowering of the pipe and to provide the compressed air.